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555 Timer IC Pin Configuration. M.S.P.V.L Polytechnic College, Pavoorchatram.

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555 timer ic pin configuration

555 Timer IC Pin Configuration

M.S.P.V.L Polytechnic College,



This integrated circuit can be used to generate stable time delays like monostable multivibrator, or it may be used as on oscillator like an astable multivibrator. It is an 8 pin dual-in-line package IC, operates with a supply voltage range of between 4.5 and 16V.


pinout diagram
Pinout diagram



The 555 Timer IC is available as an 8-pin metal can, an 8-pin mini DIP (dual-in-package) or a 14-pin DIP.

  • This IC consists of 23 transistors, 2 diodes and 16 resistors.


pin 1 grounded terminal
Pin 1: Grounded Terminal
  • All the voltages are meas­ured with respect to this terminal.


pin 2 trigger terminal
Pin 2: Trigger Terminal
  • This pin is an inverting input to a comparator that is responsible for transition of flip-flop from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.


pin 3 output terminal
Pin 3: Output Terminal
  • Output of the timer is avail­able at this pin. There are two ways in which a load can be connected to the output terminal either between pin 3 and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between pin 3 and ground
  • supply pin is called the normally on load and that connected between pin 3 and ground pin is called the normally off load.


pin 4 reset terminal
Pin 4: Reset Terminal
  • To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose, it should be connected to + VCC to avoid any possibility of false triggering.


pin 5 control voltage terminal
Pin 5:Control Voltage Terminal
  • The function of this terminal is to control the threshold and trigger levels. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform.
  • The external voltage applied to this pin can also be used to modulate the output waveform.
  • When this pin is not used, it should be connected to ground through a 0.01 micro Farad to avoid any noise problem.


pin 6 threshold terminal
Pin 6: Threshold Terminal
  • This is the non-inverting input terminal of comparator 1, which compares the voltage applied to the terminal with a reference voltage of 2/3 VCC.
  • The amplitude of voltage applied to this terminal is responsible for the set state of flip-flop.


pin 7 discharge terminal
Pin 7 : Discharge Terminal
  • This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground.
  • It is called discharge terminal because when transistor saturates, capacitor discharges through the transistor.
  • When the transistor is cut-off, the capacitor charges at a rate determined by the external resistor and capacitor.


pin 8 supply terminal
Pin 8: Supply Terminal
  • A supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).


see also
See also
  • http://www.doctronics.co.uk/555.htm#pins
  • http://iws45.iiita.ac.in/IEC2008063/Documents/555-timer-ic-working-principle.htm
  • http://www.sentex.net/~mec1995/gadgets/555/555.html
  • http://www.ecircuitcenter.com/circuits/555_timer1/555_timer1.htm